The invention relates to a method for producing high-molecular weight polyesters, such as e.g. polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polynaphthalene terephthalate (PEN), polytrimethylene terephthalate (PTT) and/or polyesters of other dicarboxylic acids and diols including copolymers thereof. The invention likewise relates to a device for implementation of this method. This concerns a single-stage tower reactor with which a method, comprising only two reactors, is made available for the total process.
Methods for continuous production of polyesters are known from the state of the art, in which multiple-stage reactor systems are used which comprise three to five different reaction vessels which are connected to each other. In the case of these methods, the polyester formation is implemented in a plurality of reaction steps which are generally configured as stirred vessels, which steps proceed spatially separately from each other: esterification, transesterification, precondensation, polycondensation and polyesterification. The reaction conditions for the esterification are thereby at temperatures between 200 and 280° C. and pressures between 0 and 4 bar, whilst the conditions for the transesterification are normally at atmospheric pressures and at temperatures between 150 and 240° C., dependent upon the initial substances, especially upon the diols. Low temperatures and low pressures are desirable for the methods in order to avoid undesired secondary reactions.
A method for continuous production of high-molecular weight polybutylene terephthalate is known therefore from DE 35 44 551 A1, in which the method is implemented at atmospheric pressure.
It is important with all the methods known from the state of the art to fulfil the following conditions:    I) Suitable process control and configuration of the devices in order to avoid undesired secondary reactions.    II) As rapid as possible removal of the byproducts, such as e.g. water, methanol, THF and acetaldehyde, as a result of which the reaction equilibrium is moved to the right and the main reaction consequently proceeds with precedence.    III) Sparing treatment, i.e. short dwell time with a minimum reaction temperature.    IV) Optimally coordinated pressure reduction and/or respectively temperature increase in order to achieve the greatest possible progress of the esterification/transesterification and also of the polycondensation/polyesterification.    V) High surface formation for improving the reaction kinetics.
These objects can be fulfilled only unsatisfactorily by means of a plurality of successively connected stirred vessels which are normally used according to the state of the art for these processes. Consequently, e.g. the quality of the product or the quantity of undesired byproducts and also the yield can be affected negatively, as a result of which, in addition to further disadvantages, such as energy consumption, maintenance requirements and investment costs, the economic efficiency of these methods is greatly impaired.
Starting from these disadvantages of the state of the art, it was the object of the present invention to provide a method for producing high-molecular weight polyesters using a single, integrated reactor which fulfils the requirements (I) to (V).
This object is achieved by the generic method with the characterising features of claim 1 and also by the generic device with the characterising features of claim 18. The further dependent claims reveal advantageous developments.
According to the invention, a method for continuous production of high-molecular weight polyester is provided, based on the esterification of dicarboxylic acids and/or transesterification of dicarboxylic acid esters with diols in the presence of catalysts with simultaneous formation of a prepolymer and polycondensation thereof into high-molecular weight polyester. The following steps thereby characterise the course of the method:    a1) A paste and/or a suspension of the dicarboxylic acids and of the diol is produced, a molar ratio of diol to dicarboxylic acid of 0.8 to 1.8 being maintained. At the same time, the temperature is maintained between 20 and 90° C. and the pressure between 0.1 and 1 bar.    a2) Alternatively to a1), a dicarboxylic acid ester can be melted and mixed with the diol in a molar ratio of diol to dicarboxylic acid ester of 1.2 to 1.8 at a temperature of 145 to 165° C.    b) The products from a1) and/or a2) are supplied continuously to a tower reactor. In the tower reactor, the esterification or respectively transesterification is then effected continuously, simultaneously and without interruption until precondensation, the following reaction conditions being maintained:    b1) The products from a1) and/or a2) are subjected to a temperature treatment at 170 to 270° C. and pressures of 0.3 to 3 bar without mechanical stirring. At the same time, a partial quantity of the catalyst and/or catalysts is supplied, whilst the reaction vapours are removed in situ from the reaction mixture.    b2) The product obtained from b1) is transferred continuously into a pipe stretch and the pressure is thereby maintained between 1 and 10 bar, the temperature between 200 and 280° C. In this reaction step, at the same time 0.03 to 0.3 mol diol are supplied.    b3) The product from b2) is transferred continuously into a third zone, a reduced pressure of 0.1 to 2 bar and a temperature between 230 and 280° C. being maintained 0.02 to 0.2 mol diol and a partial quantity of the catalyst and/or catalysts are supplied in this third step.    b4) The product from b3) is subjected to further reaction, the pressure being maintained between 0.01 and 0.1 bar and the temperature between 240 to 280° C.    b5) The reaction product water from the esterification or respectively methanol from the transesterification, the byproducts and also excess diol from the reaction steps b1) and b3) to b5) are removed and the diol is supplied back to the individual process steps after removal of low boilers.    c1) The prepolymer obtained from b4) is further processed in a normal polycondensation apparatus at temperatures between 240 and 290° C. and at pressures between 0.0002 to 0.003 bar continuously to obtain a polymer.    c2) Alternatively to c1) it is also possible to solidify the obtained prepolymer, to process it into pellets and to subject it to subsequent polycondensation in solid phase at temperatures between 160 and 230° C. under inert gas.
The new method allows the parallel, uninterrupted sequence of transesterification/esterification and precondensation of dicarboxylic acids and esters thereof with diols in a single tower reactor. As a result, the mechanical and process-technological integration of a plurality of process steps is made possible for the first time for the polyester synthesis. Preferably, the gaseous byproducts produced in step b1) and the excess diol are separated by means of a hydrocyclone in “statu nascendi” from the reaction mass. In addition, an inert gas and/or a process gas can be directed through the base to assist this separation. There is great significance in the rapid separation of low boiling reaction gases with respect to minimising the formation of byproducts by auto- and acid catalysis. The content of byproducts in the reaction material is determined, on the basis of the partial pressures of these products, by the occurring reaction pressure and the static product levels present: the higher is the total pressure, the higher is the formation of byproducts. The configuration as hydrocyclone with externally situated heat exchanger is therefore decisive for a short dwell time of the byproducts in the reaction material in the case of increasing static product level because of the auto-adjusting thermosiphon circulation, causing an immediate effective degassing in the hydrocyclone and in the heat exchanger.
During implementation of an esterification, preferably a temperature between 200 and 270° C. and a pressure between 0.3 and 3 bar is maintained in step b1). If in contrast a transesterification is implemented, then step b1) is implemented at a temperature between 170 and 200° C. and at a pressure between 0.3 to 1 bar.
In step b2), during implementation of an esterification, the pressure is maintained preferably between 2 and 6 bar, the dwell time between 1 and 5 min and the temperature preferably between 220 and 280° C. and particularly preferably between 230 to 250° C. If in contrast a transesterification is implemented, the pressure in step b2) is maintained preferably in the range between 2 and 5 bar, the dwell time between 1 and 4 min and the temperature in the range between 200 and 240° C., particularly preferably between 210 and 230° C.
In an advantageous development of the method, in step b3), the reaction product supplied continuously from step b2) is directed over a downflow cascade, the pressure falling by 20 to 60 mbar and the temperature increasing by 5 to 20° C. per reaction tray. During the esterification, a prepolycondensate is thereby produced with 5 to 20 repeat units with a conversion between 97 and 99.5%. During the transesterification, the conversion is however between 98.5 and 99.8%.
Preferably, in addition to the normal process gas, a dry inert gas and/or superheated process gas is directed into at least the first tray below the reaction material surfaces. As a result, the separation of the byproducts is assisted by “entrainment effect”, i.e. saturation of the gases. At the same time, the internal intermixing of the reaction material is achieved. The dwell time in the individual trays is preferably in a range between 5 and 15 min.
In a further advantageous development of the method, the supply of product is effected centrally in the trays. The product thereby discharges at the outer periphery of the trays distributed uniformly over the outer wall, which serves for accelerated degassing, and subsequently is directed together again centrally.
The step b4) is configured as a falling-film zone with initial pressure reduction and is implemented preferably at a temperature between 245 and 270° C. and a dwell time between 4 and 30 min, and at a pressure between 0.01 and 0.05 bar. A prepolycondensate is thereby produced with 10 to 40 repeat units with a conversion of 99.8%.
The reaction product is directed together after leaving one or more falling-film zone(s) preferably through a heated cone, a gas-liquid separation being undertaken in its central region by means of a spoiler-shaped outlet.
Preferably, 1,2-ethanediol, 1,3-propanediol or 1,4-butanediol is used as diol. The method is also suitable for cyclohexane dimethanol. Terephthalic acid is used preferably as dicarboxylic acid. During transesterification, preferably dimethyl terephthalate (DMT) is used as dicarboxylic acid ester. There are used as catalysts preferably the known tin, antimony, germanium, manganese, calcium and/or titanium metals etc., in particular as organic compounds thereof. The catalysts can also be accommodated in a porous carrier substance in order to achieve a specific closing effect.
According to the invention, likewise a device for implementing the described method according to the invention is provided, which has at least one stirring vessel for slurry preparation, reaction zones for the simultaneous esterification or transesterification and precondensation as well as at least one polycondensation reactor for the solid and/or liquid phase. The individual reaction zones are thereby combined into one tower reactor and connected at least to one reactor for the polycondensation.
The tower reactor is thereby constructed in the following manner: in the lower portion, the tower reactor is configured in the form of a hydrocyclone with an attached heat exchanger and has a supply line for the paste, the suspension and/or the liquid raw material mixture. The hydrocyclone is connected via a pressure pipe to the top side of the tower reactor. The top side of the tower reactor is thereby configured in the form of a downflow cascade. This cascade is connected via a suitable inlet pipe to the central part of the tower reactor which is configured in the form of a single- or multiple-stage falling-film zone with initial pressure reduction.
Preferably, the hydrocyclone has a vapour connection piece and is connected in the lower part of the tower reactor to a heat exchanger. As a result, it is made possible to direct the product in a natural or enforced circulation via the heat exchanger into the hydrocyclone.
In an advantageous embodiment, the heat exchanger has a separate gas chimney which leads into an upper part of the cyclone.
Preferably, the cascade has at least two, particularly preferred four trays. A stirring assembly can thereby be integrated in at least one cascade region in order to assist the input of diol or additives.
The pressure pipe is configured preferably as a double-walled jacket pipe which continues in the interior of the first top tray as a heating coil. The pressure pipe can thereby be equipped with a volume feed pump and static mixing elements or a mixing pump.
Preferably, the hydrocyclone has an inert gas inlet with a suitable gas distributor in its conical region.
In an advantageous embodiment, one of the top trays in the vapour region likewise has an inert gas inlet. The reaction gases and/or foreign gases are preferably directed from reaction zone to reaction zone through the reaction liquid in parallel flow by means of immersed supply tubes.
Preferably, the preliminary pressure reduction zone for the falling-film zone has likewise the form of a hydrocyclone which supports the flash effect and ensures suitable separation of the liquid/gas phases.
The supply of the product from the initial pressure reduction zone to the falling-film zone is effected by suitable configuration of the discharge therefrom in the concentric outer region of the falling-film zones and the product is distributed uniformly in the pipe field via channels.
It is likewise preferred that the falling-film zone has at least one pipe field. An inlet cylinder is thereby assigned to each pipe of the pipe field, which inlet cylinder ensures a uniform wetting of the inside of the pipe, the latter being equipped with overlapping, non-axial slots on the circumference, a constant filling level above the pipe series being produced because of the slot pressure loss and having a maximum overflow with an indented crown, the slots being configured such that viscosity differences cause only negligible changes in the filling level, but in fact a proportional change of filling level to liquid throughput.
It is likewise preferred that the length of the falling-film pipes is dimensioned such that total wetting is effected.
Preferably, the diameter of the falling-film pipes is selected to be larger than the largest occurring reaction vapour bubble. The reaction vapours are thereby directed in parallel flow with the downwardly flowing product.
The falling-film pipe field can preferably be used also for heat transference.
Preferably, the entire tower reactor is equipped with a jacket for heating with organic heating media in vapour form.
In a further advantageous embodiment, all the heat exchanger surfaces are equipped in the individual zones for liquid heat carriers for process-relevant temperature and heat quantity distribution.
The subject according to the invention is intended to be explained in more detail by means of the following figures and the example, without restricting the latter to these embodiments.
FIG. 1 shows the schematic structure of a tower reactor (1). A suspension of the dicarboxylic acid with the diol or the molten dicarboxylic acid ester and the diol are injected under pressure into the reaction fluid in the lower region of a heat exchanger (5) suspended on the tower reactor, the result being, due to suitable configuration of the injection connection piece (3), an optimal intermixing with the boiling reaction product in the lower part. Also a catalyst, which is advantageous for some polyester reactions, can be fed in here. The heat exchanger ensures the heating of the mixture to the boiling temperature of the reaction. The boiling reaction mixture passes via a short connection pipe which discharges tangentially into a hydrocyclone (2) for further reaction. For rapid reaction gas discharge from the reaction material, the main part of these gases is directed via a separate pipe, a vapour chimney (6), out of the heat exchanger into the gas chamber of the cyclone.
A further degassing during continuation of the reaction takes place in the hydrocyclone (2). The reacted product passes via a short connection pipe at the foot of the cyclone back into the heat exchanger (5) so that a natural circulation is produced. The entire reaction gas is withdrawn above the reaction fluid from the vapour chamber of the hydrocyclone (2).
In the case of particularly sensitive products, such as e.g. in the production of PBT, undesired tetrahydrofurane (THF) is produced by cyclisation of butanediol. The formation is increased by the presence of water which is produced for example during esterification. Advantageously, a preheated, unsaturated entrainment gas or superheated process vapour can be introduced in this case in the lower region of the cyclone, as a result of which the removal of, in particular water, methanol, acetaldehyde or tetrahydrofuran from the reaction material is accelerated.
A partial stream of the degassed reaction product is removed from the lower connection pipe and pumped by means of a pressure-increasing volume conveyor into a heated pressure pipe (4). The pressure pipe (4) is equipped at the pump outlet with a metering pipe for diol which is mixed intimately with the reaction material via static mixing elements. According to the product, a pressure here of up to 10 bar can be set via a pressure maintaining valve. Alternatively, the intermixing and the pressure can also be produced by a special mixing pump.
By introducing the diol under pressure, a spontaneous conversion with the carboxyl- or methoxy groups is initiated, which proceeds in the time range of 1 to 5 minutes and is ended after pressure reduction to atmospheric or sub-atmospheric pressure. The reaction time is adjusted in that a part of the pressure pipe is designed in the form of a heating coil in the uppermost reaction zone of the tower reactor. The double-walled jacket ensures that the reaction material cooled by pressure reduction is brought again to reaction temperature.
The pressure-reduced reaction fluid is now conducted through a downflow cascade (7) which comprises at least 2 preferably 4 to 5 reaction trays fitted with heating coils, into which the product flows centrally and dipped under the surface. The reaction gas is in contrast conducted separately out of the tray situated respectively thereabove by means of likewise dipped pipes through the reaction fluid. A differential pressure, which acts preferably as a pressure reduction from tray to tray from top to bottom is produced herefrom.
The product from the second to xth tray flows down on the outer wall, which acts as an additional evaporation surface, to each next tray in a conically shaped collector, the discharge of which is located centrally at the deepest point of the cone. In the collector, there are also located the dip pipes which introduce the reaction gas into the next tray. The trays are preferably designed for a dwell time of 5 to 10 min in order to achieve the desired reaction progress. In order to maintain the required reaction temperature, preferably a gentle increase by 2 to 10° C. per tray, each tray is equipped with heating coils and respective controls.
It results from the arrangement that the reaction gas is directed in parallel flow with the product stream, the gas bubbling through the reaction material and, on the one hand, ensuring there an optimum intermixing, on the other hand, not achieving its saturation limit because of the pressure drop with simultaneous temperature increase and consequently remaining absorbent for newly forming reaction gas (entrainment effect). A further important effect of the described downflow cascade is that the low-boiling, short-chained oligomers of the product, which are initially still available, are directed back into the reaction fluid by the reaction gas and there take part further in the reaction. Due to the introduction of the gas into the reaction fluid, by additional surface formation and contact with the gaseous diol, the bubble formation further promotes the speed of the reaction.
This is not possible for example in the case of stirred cascades and as a result the yield is reduced and interferences are caused in the subsequent condensers and vacuum devices.
Insofar as it is required for mixing additional additives or diol, an inclined stirrer (10) can be placed in the last tray, which stirring assists intermixing due to vapour bubbles.
The reaction material is brought subsequently by a suitable supply line for renewed pressure reduction into an arrangement configured similarly to a hydrocyclone, which arrangement is equipped like the previous trays with heating coils for temperature control. The gas/liquid separation is effected on the surface, being ensured by suitable baffles that the reaction fluid discharges uniformly over the indented outer edge of the tray, unaffected by forming reaction gas bubbles. The reaction fluid discharging at the periphery is collected on a pipe base plate—likewise on the periphery thereof—and is distributed on the base plate uniformly with the assistance of so-called “channels”.
The pipe base is part of a straight pipe bundle (field) (9) which serves simultaneously for film formation on the inner pipe surfaces and for heat exchange. An inlet cylinder (11) is assigned to each pipe in the bundle. Said inlet cylinder is configured with a series of non-axial, overlapping slots with a particularly designed geometry at its periphery. The geometry is established such that                a minimum level for all pipes is maintained for uniform liquid distribution,        only small level differences occur for a specific viscosity spectrum,        throughput changes effect a proportional level change only and the inner pipe surface is wetted uniformly over the entire pipe length,        the upper edge of the inlet cylinder (11) serves as an emergency overflow and is equipped with an indented crown.        
The pipe diameter is chosen such that it is greater than the largest possible occurring reaction gas bubble. The reaction vapour is directed in parallel flow with the downwardly running product film. The ratio of pipe length to pipe diameter should be between 10 and 25 and the surface of the falling-film pipes must be adapted to the wettability of the product. The product emerges as a film and/or strands on the underside of the falling-film pipes, is directed together by conical collector sheets which allow the gas flow to pass through and is supplied to a second falling-film reaction zone on the periphery. The latter is in principle configured identically to the first zone but takes into account the increased viscosities by means of corresponding measures on the inlet cylinders (11), distribution of the pipes and length of the module.
Below the module, a device is located for directing together the melt which contains in the centre a central pipe for the guidance of the reaction gases and the product. The product discharging on the device, preferably on the wall, is separated from the gas flow by a spoiler device (12), which gas flow is deflected and discharged into the gas chamber of the integrated prepolymer collector. The collected prepolymer is removed from the collector after a resting and secondary reaction time of 5 to 15 minutes via a central pipe and can now be subjected to a further treatment, e.g. granulation with subsequent solid phase or a melt phase high (post)condensation. For specific products, the possibility is provided of directing back a partial flow of the prepolymer into the lower falling-film module and of mixing it with the preproduct from the upper falling-film module so that the reaction time can be advantageously increased in a simple manner. As such, extended chain lengths can be obtained.
The outer covering of the reactor is a heating jacket which is provided preferably with a synthetic heat transfer vapour as active insulation for the heating. The temperature profile required for the reaction is produced essentially with a heat transfer oil in zones by means of the inner heating faces. The reaction gases from the different zones are discharged by normal devices, such as condensers, columns and vacuum systems, the diol being essentially directed back with small proportions of oligomer into the process.
FIG. 2 shows a further variant of the tower reactor (1) which has the essential elements as in FIG. 1.
However, in this variant of the tower reactor, a special mixing pump is used instead of the pressure maintaining valve and the static mixing elements.
Likewise in this variant, the use of an inclined stirrer for mixing additional additives and diols is dispensed.